The present invention relates to a retardation film, a polarizing plate and a liquid crystal display device.
A retardation film is used to realize a view field angle enlargement or a contrast improvement for a liquid crystal display device.
The retardation film has a specific retardation value. When the retardation value fluctuates, a view angle and a color tone vary (deteriorate), especially when being observed from a diagonal direction. Although various kinds of materials are utilized for the retardation film in order to realize various demands, there may be a case that those materials reversibly change for a fluctuation of environment (especially, humidity environment).
For example, in a case of a retardation film in which a layer (an optical anisotropic layer) to express a retardation for a support film is laminated, a TAC film conventionally has been employed as a support coated with a liquid crystal layer. However, it is well known that the TAC film has a retardation value itself (especially, a film thickness direction retardation) and the retardation value fluctuates depending on absorption and desorption of water. In particular, the absorption and desorption of water relies on hydroxyl groups of a cellulose ester. The lower a total substitution degree is, the intense more the absorption and desorption of water becomes.
On the other hand, recently, we made a cellulose derivative film to express a retardation by stretching it in place of a conventional TAC film and further realized a polarizing plate protective film additionally having a function of a retardation film by subjecting it to an alkali saponification process and laminating it onto a PVA polarizer (for example, refer Japanese Unexamined Patent Publication No. 2003-270442). However, although the fluctuation of the retardation value of the polarizing plate protective film due to humidity change is smaller than that of an ordinal TAC film, there is a situation that a further improvement is required.